4 U. S. COAST AND GEODETIC SURVEY 
0 6 12 18 0 6 12 18 24 
Feet 
Fic. 1.—Tide curve, New York, June 29-30, 1934 
relatively narrow limits as a whole. Since, however, the time of tide is only of minor 
importance in connection with the determination of tidal datum planes, this matter 
need not be pursued further here. 
The range of tide has reference to the magnitude of rise and fall of the tide. This 
varies in different localities from less than a foot at many places to more than 40 feet 
in the Bay of Fundy. Moreover, the range of tide at any given place is not constant, 
but varies from day to day, the average rise and fall being called the mean range. 
In some localities the variation from the mean range is relatively small, but in others 
the variation may be considerable. ‘This matter will be considered more fully in the 
section on variations in range. 
The type of tide has reference to the characteristic form of the rise and fall of the 
tide as revealed by the tide curve. On investigation, it is found that quite apart from 
differences in time and in range, the tides at different places exhibit striking differences 
in regard to form of tide curve. For example, in New York Harbor, the tide exhibits 
two high and two low waters in the average tidal day of 24 hours and 50 minutes, each 
rise and fall occupying a period of approximately 6 hours and 12 minutes with suc- 
ceeding tides resembling each other more or less closely. In San Francisco Bay, the 
tide likewise exhibits two high and two low waters in a day, but succeeding tides differ 
considerably. In Mobile Bay, on the other hand, the tide exhibits but one high and 
one low water in a day, each rise and fall occupying a period of approximately 12 
hours. 
Type of tide is an important matter in connection with tidal datum planes. As 
will be seen later, in the consideration of types of tides, there is a variety of forms of 
tide curves. Here it is sufficient merely to direct attention to the fact that the type 
of tide at any place is one of the important features of the tide, and together 
with the time and range constitute the principal features of the tide at that place. 
